1. Field of the Invention
The present invention relates to an antenna apparatus and a portable radio apparatus suitable for, for example, portable cellular telephones.
2. Description of the Related Art
In a typical conventional portable cellular telephone, as illustrated in FIG. 1, openings 12h and 13h for a receiver (speaker) and a transmitter (microphone), respectively, are provided on the front surface 10f of a telephone housing 10. Also, disposed on the front surface 10f are dial keys 14 and various types of function control keys 15. A liquid crystal display 16 is further located at the central portion of the front surface 10f.
An arcuate projection 10c is formed on the upper left end of the housing 10 so that the housing 10 can be prevented from be dropped during use. A function-controlling operational knob 17 is disposed in the vicinity of the projection 10c. The user holds the housing 10 with his/her left hand and moves the peripheral end of the knob 17 upward or downward with the thumb so as to rotate the knob 17 clockwise or counterclockwise. A flexible monopole antenna, a so called whip antenna 18, having a length equal to one fourth of the wavelength .lambda. of the transmitting and receiving frequency, is mounted on the top surface 10t of the housing 10 in such a manner that it can be accommodated in the housing 10. The whip antenna 18 can be pulled out of the housing 10 to use the cellular telephone.
However, when the whip antenna 18 is accommodated within the housing 10, the input impedance increases, since the whip antenna is located near a ground conductor. It is thus difficult to achieve impedance matching. The increased input impedance seriously decreases the antenna gain due to impedance mismatching.
One of the measures to improve the antenna gain when the movable whip antenna is stored within the housing is to use a top-load whip antenna obtained by directly connecting a helical antenna having an electrical length substantially equivalent to one fourth of the wavelength (.lambda./4) to the forward end of a monopole antenna. In this type of antenna, when the whip antenna is pulled out of the housing, power is supplied to the base end of the whip antenna through a fixed feeder contact point so that it is operable as a single antenna formed of a combination of the helical portion and the monopole portion. When the whip antenna is accommodated within the housing, on the other hand, power is supplied to the forward end of the antenna to principally operate the helical portion, thereby obtaining a satisfactory level of the gain.
When the top-load whip antenna constructed as described above is stored in the housing, the monopole portion, which does not contribute to radiation, disadvantageously serves as an open stub, which produces an adverse influence on the input impedance of the antenna, thereby disturbing the impedance matching. An undesired electrical connection is also caused between the monopole portion and a high-frequency circuit.
To further overcome the drawbacks caused when the above type of top-load whip antenna is stored within the housing, another type of antenna has come into use: that is, a composite antenna in which the helical portion and the monopole portion are mechanically connected but electrically separated from each other when accommodated within the housing.
An example of a conventional complex antenna construction will now be explained with reference to FIGS. 2 and 3. A tubular antenna-mounting sleeve 101 is attached to a synthetic-resin-formed housing 10 for a portable radio by such means as insert-molding. A tubular antenna-supporting sleeve 102 is fit into the sleeve 101 by such means as a screw thread. A circuit board (printed board) 21 is arranged within the housing 10. Mounted on the circuit board 21 are the circuits required for this portable telephone, such as a transmitting circuit (receiving circuit) represented by a signal source 22, a matching circuit 23, and the like. A power-feed spring 24 is connected to the matching circuit 23 and is also in contact with the antenna-mounting sleeve 101.
The composite antenna apparatus comprises a helical antenna structure 30 and a monopole (whip) antenna structure 40. The helical antenna structure 30 is largely formed of a helical conductor 31 having a substantially .lambda./4 electrical length. The conductor 31 is wound at one end around the top portion of a support conductor 32 having a T-shaped cross section, and is completely stored in the longitudinal direction in a protective casing 33 formed of a required insulating material. The monopole antenna structure 40 is also primarily formed of a linear conductor 41 having a substantially .lambda./4 electrical length. The overall structure 40 is substantially covered with an appropriate dielectric layer 42 in the longitudinal direction. The conductor 41 is mechanically connected at its top end to a tubular separator 51 formed of a required insulating material. The conductor 41 is connected at its bottom end to a tubular support conductor 43. A larger stopper sleeve 44 is further coupled to the bottom end of the support conductor 43.
The top end of the separator 51 of the monopole antenna structure 40 is mechanically coupled to the bottom end of the support conductor 32 of the helical antenna structure 30, whereby the helical antenna structure 30 and the monopole antenna structure 40 can be mechanically integrated to each other.
The diameters of the support conductors 32 and 43 of the helical antenna structure 30 and the monopole antenna structure 40, respectively, are set to equal the internal diameter of the antenna-supporting sleeve 102. The support conductors 32 and 43 are slidably held by the sleeve 102. With this construction, as illustrated in FIG. 2, when the linear conductor 41 is pulled out of the housing 10, the support conductor 43 of the monopole antenna structure 40 becomes engaged with the supporting sleeve 102 so as to supply power to the conductor 41 from the signal source 22 through the matching circuit 23, the power-feed spring 24 and the sleeve 101 whereby the linear conductor 41 can serve as a radiating conductor.
On the other hand, as shown in FIG. 3, when the conductor 41 is stored within the housing 10, the support conductor 32 of the helical antenna structure 30 comes into contact with the supporting sleeve 102 so that power can be supplied to the helical conductor 31 from the signal source 22 in a manner similar to the operation described above. The helical conductor 31 can thus function as a radiating conductor.
An explanation will further be given of another example of conventional composite antenna construction. In this example shown in FIGS. 4 and 5, the support conductor 32r of the helical antenna structure 30 is formed in a ring-like shape and mounted on the top ends of the sleeve 101 and the supporting sleeve 102 of the housing 10 across a spacer 103 formed of an insulating material. The helical antenna structure 30 can thus be secured to the housing 10. An opening 33o is provided in the top surface of the protective casing 33. On the other hand, a tubular connecting conductor 52 is coupled to the top end of the separator 51 connected to the top end of the conductor 41 of the monopole antenna structure 40. A member 53 doubling as a stopper and a knob formed of a required insulating material and having a T-shaped cross section is further coupled to the top end of the connecting conductor 52.
The diameters of the support conductor 43 of the monopole antenna structure 40 and the connecting conductor 52 are determined to be equal to the internal diameter of the antenna-supporting sleeve 102. The support conductor 43 and the connecting conductor 52 are slidably held by the sleeve 102.
One end of the monopole antenna structure 40 is inserted into the protective casing 33 of the helical antenna structure 30 through the opening 33o so that it can be disposed coaxially with the helical antenna structure 30 while being mechanically independent. The other elements of the apparatus are constructed in a manner similar to those shown in FIGS. 2 and 3.
Accordingly, as shown in FIG. 4, when the linear conductor 41 is withdrawn from the housing 10, the support conductor 43 of the monopole antenna structure 40 is brought into contact with the support sleeve 102. This allows power to be supplied to the conductor 41 from the signal source 22 through the matching circuit 23, the power-feed spring 24 and the antenna-mounting sleeve 101, whereby the conductor 41 can serve as a radiating conductor. In contrast, as illustrated in FIG. 5, when the conductor 41 is stored within the housing 10, the support conductor 32r of the helical antenna structure 30 becomes engaged with the supporting sleeve 102 across the connecting conductor 52. Thus, power is supplied to the helical conductor 31 from the signal source 22 through the matching circuit 23, the power-feed spring 24 and the sleeves 101 and 102 so that the conductor 31 can function as a radiating conductor.
In the field of mobile communications, the number of available frequencies is decreasing due to improvements in the quality of communications and an increase in the capacity of lines. This necessitates the use of a frequency band exceeding 1 GHz. In response to such a high-frequency band, for example, 1.9 GHz, the length of the above-described monopole antenna having a .lambda./4-electrical length is required to be about 4 cm. Even though it is pulled out of a cellular telephone, such a short monopole antenna is too short to project sufficiently away from a user's body, in particular, the user's head. This adversely affects the radiation characteristics of the antenna, such as a resulting decrease in the antenna gain. Further, the .lambda./4-electrical-length helical antenna has a shorter axial length than the monopole antenna. The above-described adverse influence caused by a user's head is thus accentuated, thereby further reducing antenna radiation characteristics to a greater level.